1. Field
An input device having an electrostatic sensor is provided. The input device includes an electrostatic sensor that outputs an electrical signal by approach or contact of a part of a human body, such as a hand or a finger.
2. Related Art
In general, an input device having an electrostatic sensor for detecting change in capacitance is widely used for various electronic apparatuses. For example, in electronic apparatuses that recently came into wide use, such as mobile telephones or personal digital assistants (PDA), an input means that inputs information, such as numeral or character, is employed.
An electrostatic sensor of an input device is generally provided at the backside of an input surface of an input key formed on an electronic apparatus. This electrostatic sensor includes a substrate having a flat electrode. When a hand or a finger of an operator of the electronic apparatus approaches or makes contact with the input surface of the input key facing the electrode, the capacitance of the electrode is changed and an output signal according to the change is transmitted to a control means of an input device, thereby performing an input operation of the input device.
As shown in FIG. 10, a conventional input device 101 generally includes a clock signal generating means 104 for generating a clock signal Ck, a delay means 105 having the above-described electrostatic sensor 102, a logical product means 106 connected to the clock signal generating means 104 and the delay means 105, a smoothing means 107 for smoothing an output signal obtained from the logical product means 106, an A/D converter 108 for converting the smoothed output signal from analog to digital, and a control means 109 for controlling an input operation based on the digital output signal. The delay means 105 is a delay circuit including an electrode 102 and a resistor R formed on a dielectric substrate, the logical product means 106 is a general AND circuit, the smoothing means 107 is a smoothing circuit including a resistor R and a capacitor c, and the control means 109 is a control circuit including a central processing unit (CPU) or a memory (see Patent Document 1).
FIG. 11 shows a detection operation of the conventional input device 101 from a left side to a right side in time series. As shown in FIG. 11, when a finger H of a human body is sufficiently separated from the electrostatic sensor 102 (T0), a delay signal S0 having the same wavelength as that of the clock signal Ck is output from the delay means 105. Since a time for reaching a value higher than a threshold value Vt2 of the delay signal is long, a pulse width t0 of an output signal P0 of the logical product means 106 output based on the clock signal Ck and the delay signal S0 is substantially equal to the pulse width of the clock signal Ck, and an output signal A0 smoothed by the smoothing means 107 does not cause a voltage drop. To this end, although the output signal A0 passing through the smoothing means 107 and the A/D converter 108 is input to the control means 109, the control means 109 determines that the input operation is not performed (hereinafter, a distance, in which the finger H of the human body is sufficiently separated from the electrostatic sensor 102, is referred to as “non-reaction distance” and a time when the finger H of the human body approaches the electrostatic sensor 102 up to the non-reaction distance D0 is referred to as “non-reaction time”).
Subsequently, when the finger H of the human body approaches the electrostatic sensor 102 closer than the non-reaction distance D0 (T1), the wavelength of the clock signal Ck is changed to a delay signal (RC delay signal) Sb of which the wavelength slowly rises and the delay signal Sb is output from the delay means 105. Since a time for reaching a value higher than the threshold value Vt2 of the delay signal is shorter than T0, a pulse width tb of an output signal Pb of the logical product means 106 output based on the clock signal Ck and the delay signal Sb becomes smaller than a pulse width tc of the clock signal Ck (tc>t0>tb). In addition, an output signal Ab smoothed by the smoothing means 107 causes a voltage drop from V0 to Vb such that the output signal does not fall below a threshold value Vt3. To this end, although an output signal Ab passing through the smoothing means 107 and the A/D converter 108 is input to the control means 109, the control means 109 determines that the input operation is not performed (hereinafter, a distance, in which the electrostatic sensor 102 reacts, is referred to as “reaction distance” and a time when the finger H of the human body approaches the electrostatic sensor 102 up to the reaction distance Db is referred to as “reaction time”).
In addition, when the finger H of the human body approaches or makes contact with the electrostatic sensor 102 closer than the reaction distance Db (T2), the clock signal Ck is changed to a delay signal Sa of which the wavelength rises slower than that upon the reaction time T1 and the delay signal Sa is output from the delay means 105. In this case, since a time for reaching a value higher than the threshold value Vt2 of the delay signal is shorter than T1 upon the reaction time, a pulse width ta of an output signal Pa of the logical product means 106 output based on the clock signal Ck and the delay signal Sa significantly becomes smaller than a pulse width tc of the clock signal Ck (tc>t0>tb>ta). In addition, an output signal Ab smoothed by the smoothing means 107 causes a voltage drop from Vb to Va such that the output signal falls below the threshold value Vt3. To this end, when an output signal Aa passing through the smoothing means 107 and the A/D converter 108 is input to the control means 109, the control means 109 determines that the input operation is performed (hereinafter, a distance, in which the electrostatic sensor 102 reacts and the input operation was performed, is referred to as “input distance” and a time when the finger H of the human body approaches the electrostatic sensor 102 up to the input distance Da is referred to as “input time”).
Furthermore, when the finger is separated from the input distance Da to the reaction distance Db (T3), the same delay signal Sb and output signal Pb as those upon the reaction time T1 are generated, and, when the finger is separated from the reaction distance Db to the non-reaction distance D0 (T4), the same delay signal S0 and output signal P0 as those upon the non-reaction time T0 are generated.
The conventional input device 101 determines whether the input operation of the input key having the electrostatic sensor 102 is performed, according to the change in capacitance generated when the finger H of the human body approaches or makes contact.
[Patent Document] Japanese Unexamined Patent Application Publication No. 2004-201175
However, when a noise source such as a high-frequency power supply is provided in the vicinity of the electrostatic sensor 102 or broadcasting wave is spread in the vicinity of the electrostatic sensor 102, that is, the electrostatic sensor 102 is provided in an environment which is apt to be influenced by the electric wave, although a portion of the human body approaches or makes contact with the electrostatic sensor 102, the capacitance of the electrostatic sensor 102 is changed and thus malfunction of the input device 101 may be caused.
For example, as shown in FIG. 12, when the capacitance of the electrostatic sensor 102 is changed by noise Nh spread in the vicinity of the electrostatic sensor 102 upon the non-reaction times T0 to T4 (T1 to T3), although the finger H of the human body does not approach or make contact with the electrostatic sensor 102, the same delay signal Sa as that upon the input time is transmitted to the logical product means 106. Thus, the logical product means 106 transmits the same output signal Pa as that upon the input time to the smoothing means 107. Accordingly, the conventional input device 101 may actually determine that the input operation was performed, although the finger H of the human does not approach or make contact with the electrostatic sensor 102. Thus, malfunction may be caused.
In addition, as shown in FIG. 13, when the capacitance of the electrostatic sensor 102 is changed by noise N spread in the vicinity of the electrostatic sensor 102 (T1 to T3), the same delay signal Sb as that upon the reaction time is transmitted to the logical product means 106. Thus, the logical product means 106 transmits the same output signal Pb as that upon the reaction time to the smoothing means 107. To this end, the smoothed output signal Ab drops from the voltage V0 to the voltage Vb. Accordingly, since the output signal Aa smoothed upon the reaction time, which is the same as that upon the input time, drops to the voltage Va lower than the threshold value Vt3 (T2), the conventional input device 101 may determine that the input operation is performed at the input distance Da, although the finger H of the human approaches or makes contact with the electrostatic sensor 102 up to only the reaction distance Db. Thus, malfunction may be caused due to change in sensitivity of the electrostatic sensor 102.